Currently, a WiBro or WiMax system that is established or promoted as a domestic or foreign portable Internet standard is based on an orthogonal frequency division multiplexing (OFDM). The OFDM is a multi-carrier modulation scheme using a plurality of orthogonal subcarriers. An orthogonal frequency division multiple access (OFDMA) provides the multiplexing of multi-users by combing the OFDM with frequency division multiple access (FDMA) or time division multiple access (TDMA).
As used herein, the term “downlink” refers to the communication link from a base station to a mobile station, and the term “uplink” refers to the communication link from the mobile station to the base station.
In a wireless communication system based on the OFDMA, a base station provides services for a plurality of mobile stations. Uplink signals transmitted from the plurality of mobile stations can arrive at the base station, respectively, with different delay times and transmitting powers from each other. In this case, the uplink signals may not maintain orthogonality between subcarriers, which results in a severe signal loss.
One of methods for adjusting the time delay and transmitting power between a base station and a mobile station is to employ a ranging channel. A group of subcarriers is allocated to the ranging channel. The base station estimates a transmission delay and receiving power of the mobile station by using the ranging channel.
The ranging channel includes a ranging code. The ranging code is selected among predetermined set of pseudo-noise ranging codes.
A variety of methods are used in order for the base station to estimate the transmission delay of the mobile station using the ranging channel. The base station transforms a radio frequency (RF) signal transmitted from the mobile station into a baseband signal. The base station transforms the baseband signal into a frequency domain signal through a Fast Fourier Transform (FFT). The base station extracts the ranging channel from the frequency domain signal. Then, the base station multiplies the ranging channel by a phase rotation component (e−j2πkτ/N) generated by an arbitrary time delay (τ) to eliminate the phase component of the ranging channel due to time delay. The base station estimates the transmission delay using correlation between the ranging channel removed of the phase component and the predefined set of ranging codes.
The ranging code included in the ranging channel must be multiplied by the phase rotation component one by one, which requires a large quantity of calculations. For example, if an arbitrary time delay it is wished to observe is a P sample interval and a group of ranging codes is composed of S codes, it is required that the ranging code should be subjected to a phase rotation component eliminating process P times and correlation should be obtained S times. In addition, the generation of the phase rotation component from the baseband signals contributes to an increase in the quantity of calculation of hardware.
Therefore, there is a need for a method of allowing the base station to efficiently and rapidly estimate a transmission delay and a transmission power of the mobile station by using the ranging channel.